The present invention relates to a method and apparatus for irradiating low-energy electrons, and in particular to a method of forming a low-energy electron beam used in a process such as neutralization of a charged status created by processing a semiconductor wafer, conversion of positive ions into negative ions in order to process a surface of a semiconductor substrate.
In general, a charged beam apparatus irradiates ions or an electron beam onto an insulated sample such as a semiconductor wafer or a non-grounded floating sample (hereinafter simply called "sample"), to either enable analysis using secondary electrons or secondary ions emitted from the sample's surface, or to use the ion beam for ion implantation or ion etching of the semiconductor wafer's surface. If a positive ion beam is irradiated onto the sample, the sample builds up a positive charge from the incident positive ions or the secondary electrons generated by the ion beam irradiation. This positive charging of the sample causes various problems in the use of ion beam devices. One example of a problem caused if the ion beam device is used for mass spectroscopy is described below.
Secondary ion mass spectroscopy (SIMS) uses a method by which, in a pre-step of the mass spectroscopy, the energy of secondary ions is analyzed and only secondary ions of a certain energy level are subjected to the mass spectroscopy, in order to ensure highly accurate mass spectroscopy. However, charge on the sample's surface caused by the above described positive ion beam irradiation will change the positiveness of the sample's voltage, and/or generate an electric field in the vicinity of the surface of the sample. Changes in the energy distribution of the secondary ions caused by this charge will reduce the transmissivity of the energy distribution, and thus reduce the detection efficiency. Charge on the sample is a major cause of variations in detection efficiency in SIMS.
Another example of the problems occurs when the ion beam irradiation apparatus is used for semiconductor devices. An ion implantation apparatus that irradiates a large-current ion beam at the semiconductor devices is used, but if the charge on the semiconductor wafer is too large, the insulation will break down and the semiconductor devices will be damaged.
Similar problems occur with an ion beam etching apparatus that maintains an ion source at a positive charge, generates a plasma using an inert gas, and draws inert gas ions from the ion source to irradiate them onto a wafer to etch the surface of the wafer; an ion beam sputtering device that draws out an ion beam accelerated at a high energy level from an independent ion source and bombards it onto a target placed in high-vacuum atmosphere, to form a layer on a sample such as a semiconductor wafer in a low-pressure gas at 0.01 Pa or less; an ion plating device that ionizes or activates vaporized atoms from a vaporization source activated by a glow discharge to accelerate them, then bombards them onto a semiconductor wafer placed on a cathode side to form a thin layer thereon by deposition; or a cluster ion beam deposition device that ionizes a cluster of 100 to 1000 atoms in a hard vacuum, accelerates the cluster, and bombards it onto a wafer to form a thin layer by deposition. After one of these devices has been used, the sample such as a semiconductor wafer is charged, and the same insulation damage as that described above can occur.
For this reason, when an ion beam device is used, it has become necessary to use a method designed to neutralize the charge by additionally irradiating a negatively charged electron beam onto the charged sample surface, in order to relieve the positive charge on the sample's surface.
A conventional method of electron beam irradiation using this form of charge neutralization is illustrated in FIG. 9. First, an ion beam 901 having a positive charge is irradiated onto a wafer 902 by an ion beam apparatus (not shown in the figure). Either the ion beam itself, or the beam of secondary electrons generated by the ion beam, positively charges the irradiated surface. This portion is a charged portion 909. Electrons 905 are irradiated onto the sample's surface in order to neutralize the charged portion 909. To generate these electrons, a power source V.sub.F 906 applies a voltage to a cathode 903 to heat it. The electrons 905 emitted from the cathode 903 are accelerated by an acceleration voltage from a voltage source V.sub.A 908 applied between the cathode 903 and the wafer 902, and are irradiated onto the charged portion 909 on the wafer 902. A reflection plate 904 is provided around the cathode 903 to suppress scattering of the electrons 905 and direct them, and a voltage source V.sub.R 907 is connected between this reflection plate 904 and the cathode 903 to negatively bias the reflection plate 904 with respect to the cathode, and thus the positive charge on the charged portion 909 is neutralized by the electrons 905.
In the surface processing of a wafer made of a substance such as silicon, neutral activated species or positive ions are used. For example, in reactive ion etching (RIE), a reactive gas plasma is formed by high-frequency discharge, a wafer surface is exposed to this plasma so that a self-bias voltage (Vdc) is induced in the surface, positive ions are drawn out from the plasma, and active species that have been adsorbed into the wafer surface are bombarded by the ions to etch the surface. The ion energy is usually at least 100 eV, and it could even be several 100 eV. Under this ion bombardment, since the wafer being etched is bombarded by ions at an energy far higher than the binding energy of the wafer, material-specific selectivity is difficult, and often the material that should be left behind is etched away as well. On the other hand, when a neutral reactive species is used, as in chemical dry etching, there is absolutely no ion bombardment because the reactions are all chemical, so that by choosing a suitable combination of material and gas it is possible to etch with a high degree of selectivity. With this kind of etching, it is possible to ensure that only silicon is etched, leaving silicon dioxide virtually untouched. However, it is extremely difficult to do the opposite and etch only silicon dioxide, leaving silicon untouched. This is because the Si-0 bonds are stronger than the Si-Si bonds, so the etching speed of silicon dioxide is less than that of silicon.
To insulate the elements on a semiconductor substrate such as silicon, the LOCOS method is usually used. With this method, a mask pattern of a substance such as a nitride film is formed on the wafer surface and the wafer is exposed in an oxidizing atmosphere so that a thick oxide film forms on the parts of the wafer surface not covered by the mask. With this method, the oxide film is eaten away by a phenomenon called bird's beaks at the boundary between the wafer and the mask material, so the oxide portions must be made unnecessarily large, destroying the high levels of integration.
Recent thinking has suggested using negative ions such as NF.sub.3.sup.- ions to improve selectivity in this etching. In other words, atoms or molecules of a reactive gas are given a negative charge, and the semiconductor substrate being processed, such as a wafer, is exposed to these ions.
A material that tends to form chemical bonds with the negative ions of the reactive gas, such as a silicon oxide film, has a far greater adsorption that a material that does not have this tendency, such as silicon, so that the adsorbency efficiency of the etching species is greater and therefore the etching speed is relatively higher. This enables highly selective etching of substances such as oxide films, and also the use of negative oxygen ions during the formation of oxide films. For example, a silicon semiconductor substrate can be heated to approximately 800.degree. C., and a bias voltage of about 10 V can be applied to draw out negative ions. A silicon oxide film can be formed on a substrate in this state by placing it into an atmosphere of negative oxygen ions. This means that the negative oxygen ions drawn to the boundary formed by the bias voltage are diffused in the lengthwise direction on the semiconductor substrate, so the oxide can be given directionality, thus reducing bird's beaks. Similarly, the diffusion that occurs in impurity diffusion processing can be given directionality by forming the above oxide film by turning atoms of an impurity such as phosphorus, boron, or arsenic into negative ions, then heating the semiconductor substrate while a bias voltage is applied to it.
However, when electrons are irradiated onto a charged portion with a conventional apparatus such as that described above, in order to neutralize the charge, the problem arises that electrons can be oversupplied. In this case, the charge on the wafer surface becomes even more negative, and the surface potential on the wafer falls until it is the same as the energy of the electrons irradiated onto the wafer. For example, if electrons are oversupplied when the energy of the electrons irradiated onto the wafer is assumed to be a maximum of 100 eV, the potential of the wafer surface will continue to fall until it is -100 V.
In the conventional apparatus shown in FIG. 9, the hot electrons 905 accelerated by the voltage source V.sub.A 908 are irradiated onto the wafer. The voltage of the voltage source V.sub.A 908 is normally set to between -100 V to -500 V. If electrons of an energy of 100 eV or greater are irradiated onto the wafer, excess electrons will be supplied and the potential of the wafer surface will drop to negative. This drop in the potential of the sample's surface will shift the energy distribution of the secondary ions (positive ions) emitted from the sample's surface toward the low-energy side, by an amount equal to the surface potential. The shift will be toward the high-energy side for negative secondary ions or secondary electrons. As a result, if the energy distribution of the secondary ions or electrons used in SIMS has shifted, the transmissivity of an energy filter in a subsequent stage will greatly drop. To prevent this phenomenon, it is necessary to control the amount of electrons supplied for the neutralization, but with conventional devices there is no way to monitor the charge on the area irradiated by the ion beam, so it is extremely difficult to control the amount of irradiation electrons to correspond to the amount of charge.
A method is known of selectively irradiating low-energy electrons onto the charged portion in order to remove the supply of excess electrons (Japanese Patent Laid Open No. 63-257175 (1988)). With this method, when hot electrons hit the target, reflected electrons that fly off the target or are amongst the secondary electrons are removed, and only the secondary electrons are irradiated. Since the reflected electrons are at a high energy, removing them ensures that low-energy electrons are irradiated, so that oversupply of electrons can be prevented. However, electrons that have flown off the target have a large energy distribution and they also scatter over a wide range, so if this method is used as is it is not very efficient, even if the reflected electrons are removed. In addition, with this method, hot electrons hit perpendicular to the target's surface, but this means that the concentration of electrons at the center is directed back in the direction from which the electrons were generated, so that only a very small amount of the secondary electrons are actually used.
In ion etching, negative ions that can etch both efficiently and selectively can be formed by irradiating radical ions of low-energy electrons, but it is extremely difficult to efficiently control this energy. The negative ions are formed by introducing a gas such as NF.sub.3, ClF.sub.3, Cl.sub.2, or F.sub.2 into a discharge tube, then cracking it with some means such as microwave discharge. After the cracking, the atoms such as fluorine or chlorine that are reactive gas components generated by the cracking are supplied to a vacuum chamber. When these atoms are irradiated by a low-energy electron beam, negative ions of fluorine or chlorine are formed. The energy of the electron beam used to negatively ionize these atoms is set to be maintained lower than the energy needed to ionize the atoms, so that no positive ions are generated. This value is approximately 10 to 15 eV, regardless of the atoms, which means that if the energy of the electron beam used to negatively ionize the atoms is held to below about 20 eV, the amount of positive ions generated can be kept extremely low. Even when the reactive gas is in molecule state, the necessary energy is virtually the same as that for atoms. Creating the low-energy electron beam necessary for this negative ionization is difficult, and controlling this energy is also a problem.